1. Field of the Invention
The present invention relates to a hard disk device (hard disk drive apparatus) with a thermally assisted head and especially relates to a driving current control mechanism of a laser diode.
2. Description of the Related Art
In accordance with high recording density of a hard disk device, performance improvement in a thin film magnetic head is demanded. As the thin film magnetic head, a composite type thin film magnetic head is widely utilized that has a configuration in which a reproducing head having a magneto resistance effect element (MR element) for reading and a reading head having an inductive electromagnetic transducer for writing are laminated.
A recording medium for magnetic recording is made from a discontinuous medium where magnetic micro particles are assembled, and each magnetic micro particle has a single magnetic domain structure. Since a recording area (each bit) is composed of a plurality of the magnetic micro particles, the recording area has an asperity-shaped-boundary. In order to enhance the recording density, the asperity of the boundary of the recording area has to be reduced. For that, it is effective to reduce the size of the magnetic micro particles; however, if the magnetic micro particles are reduced in size, thermal stability is reduced due to the decrease in volume of the magnetic micro particles. In order to increase the thermal stability, it is preferred to utilize a magnetic material having a large magnetic anisotropy constant Ku; however, it becomes difficult to record information with a conventional magnetic head because coercive force of the recording medium increases when anisotropy energy of the magnetic micro particles is increased. To solve this problem, a method is proposed in which recording is performed when coercive force is reduced by simultaneously applying a magnetic field and heat at the time of recording. Such a method is referred to as thermally assisted magnetic recording. The thermally-assisted magnetic recording is similar to optical magnetic recording; however, the space resolution is realized by light in the optical magnetic recording, on the other hand, the space resolution is realized by a magnetic field in the thermally assisted magnetic recording.
As an example of such thermally assisted magnetic recording, in the specification of U.S. 2010/0202081, a thermally assisted head that includes a surface-emitting laser diode and a photodiode is disclosed. Laser light emitted from the laser diode is introduced into a waveguide. A plasmon antenna disposed at a tip of the waveguide generates near field light, and a recording medium is heated. Simultaneously, magnetic flux is supplied from a main pole of a recording part to the recording medium, and information is recorded to the recording medium while coercive force is reduced. A portion of the laser light emitted from the surface-emitting laser diode is detected by the photodiode.
Generally, characteristics and especially light strength of laser diodes are highly variable. Even when the same driving current is applied, the light strength widely varies for each laser diode. In the case of a shortage of light strength, it is difficult to record information because coercive force is not sufficiently reduced. In the case of excessive light strength, an area where temperature increases due to the laser light widens, and a track width is increased, resulting in restriction of high density recording. Therefore, an adjustment, which will be described below, may be performed to suppress effects due to the variation of the light strength of the laser diode.
Specifically, before shipping the hard disk device from a factory or at a first stage at which a user uses the hard disk device, the hard disk device “learns” the most appropriate recording power. There is an example of a learning process in which a relationship between output current of the photodiode and a signal-to-noise-ratio (SNR) is acquired. The output current of the photodiode is the light strength that the photodiode monitors while increasing the driving current of the laser diode, and the SNR is a ratio of signal with respect to noise of a reproducing signal. More specifically, information is recorded to a recording medium while increasing the driving current of the laser diode, and the recorded information is read by a reproducing element. The reproducing element generates a reproducing signal current that corresponds to the recorded magnetic information. By analyzing the reproducing signal current, the SNR is calculated and determined.
By increasing the driving current of the laser diode, the SNR of the reproducing signal current reaches a saturated point. As described above, the output current of the photodiode that is defined where the SNR reaches the saturated point is learned and is stored in a memory. During use of the hard disk device, the driving current to be applied to the laser diode is controlled to obtain the learned output current of the photodiode. By determining the recording power with such a method, the variation of the light strength among laser diodes is suppressed, and it becomes possible that a sufficient SNR is obtained for every laser diode. Also, when the output current of the photodiode at the saturated point is used to control the driving current of the laser diode, application of excessive driving current to the laser diode is prevented.
However, an environment of the hard disk device, and specifically an internal temperature of a housing of the hard disk device, varies not only by the installation environment of the hard disk device itself but also by operation condition of the hard disk device. Specifically, as the internal temperature of the housing varies, the required recording power varies. Therefore, when the “learned” output current of the photodiode is used to control the driving current of the laser diode and when the internal temperature of the housing at the time of learning and the internal temperature of the housing at the operation time widely vary, it may not be possible to obtain the appropriate recording power.
It is an object of the present invention to provide a hard disk device that may heat the recording medium at a proper temperature even when the internal temperature of the housing varies, and that includes a thermally assisted type recording element. Also, it is an object of the present invention to provide a control method for such a hard disk device.